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forklord

07 YZ450f Jetting/exhaust

5 posts in this topic

So I rejet with a dynojet kit with 160 main and new needle in same position. says its for aftermarket exhaust below 3000ft. I dont have the exhaust on yet but even with the fuel screw out most of the way it still crackles/pops as you slow down from mid-low rpms. high rpms have no popping. but i was thinking that if i put a slip on exhaust on my bike it will make it run even more lean, more than it is now. this jet should make it richer no doubt? i also have a size larger pilot jet ordered just in case. its about 5 degrees celsius outside. and the last time i ran it before this it was 10 degrees or so and it ran slightly better. thoughts?

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To compare, I have an 06 with the JD kit and at around 1300ft main jet is 170 and pilot is 48, needle is changed also, but I cant remenber the numbers on it at this time, and it had a white brothers full exh. I change back to the stock pipe setup and the main is fine, I played with the pilot from 42, 45 and it was best with the 48, air screw at 2. The bike is run on motocross and some wood trails and it is working good. Just some info for you to think about.

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Here we go again with the angst over decel backfire.

First, you need to understand that a correctly jetted, healthy YZ450 should crackle a bit on decel. If it doesn't, it's probably too rich, and won't run as well as it should. It's simply the nature of the engine.

The two most common causes of excessive decel backfire are lean idle circuit jetting and air leaks in the exhaust system. If you have an engine with a leaky exhaust, and pursue a correction to the backfire issue with jetting, you will end up with a bike that starts hard, especially when hot, fouls plugs, idles poorly, and stalls on a throttle chop at low speeds. It's always a good idea to check for a leaky exhaust before fiddling with the jetting or idle mixture.

If it's jetting related, it is nearly always the fault of the pilot (idle) circuit, as the main circuit is not active when the throttle is closed. If you are certain of your exhaust, and that there are no other causes of a lean idle condition, such as air leaks, or tight intake valve clearance, proceed with adjusting the pilot jet, and replacing it if need be:

http://www.thumperfaq.com/jetting.htm#PJ

http://www.thumpertalk.com/forum/showthread.php?t=327405

Once you have the pilot set tso that the engine runs correctly, you can see what sort of backfiring it does from that point. If it still seems excessive, you can tweak things a little.

Bear in mind that this can also be the result of the engine running too rich, but this is much less common.

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thanks gray racer. same thoughts i had pretty much. i think half the problem may be i tried to fit my aftermarket exhaust on only to realize it was for an 06 not an 07 so i put my stock pipe back on and therefor possible leaks as my bike was running slightly better before i removed the exhaust for the first time ever. im wondering if over tightening the clamp from the header to midpipe may cause a leak as i didnt torque. I just reefed on it heh. ill wait for my pipe to come in the mail, then try a larger pilot if needed, and then needle position if needed as well and post it here. thanks!!

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